1. Field of the Invention
The present invention relates to a porous solid for adsorption separation of components in gases, and to an adsorption separation process employing it for components in gases.
2. Related Background Art
Zeolite, mesoporous substances, silica gel, alumina and the like are among the porous substances of the prior art utilizable for adsorption and separation of components in gases, and each has their unique features, which are described below.
Zeolite-based crystalline porous solids are utilized in a wide range of industrial fields as catalysts and ion-exchangers because of the physical and chemical surface properties of the porous solids. Since zeolite-based crystalline porous solids have an extremely fine uniform structure with a pore size of 0.3-1.3 nm, they exhibit an excellent function in terms of highly selective catalytic reactivity for low molecular weight compounds, etc. and excellent adsorption properties.
The crystalline mesoporous substance (mesoporous solid) with pores in the meso range of 2-50 nm described in Japanese Unexamined Patent Publication HEI No. 8-67578 and elsewhere has a high surface to volume ratio and excellent homogeneity of the porous structure, and it can therefore be used as a selective catalyst or adsorbent for high molecular weight compounds, i.e. compounds with a molecular size approximately equivalent to the pore size, while the molecular diffusion rate is also excellent due to the sizes of the pores.
Amorphous porous solids of silica gel or alumina are usually prepared by sol-gel methods, and structural control can be easily accomplished by using various organic substances as templates. Because such amorphous porous solids have a wide pore size distribution from micropores to mesopores to macropores, the presence of the macropores allows their greatly extended function as an adsorption carrier, while their wide pore size distribution allows their use as adsorption carriers and reaction catalysts for an extremely wide range of organic compounds, from low molecular weight to high molecular weight. Japanese Unexamined Patent Publication HEI No. 9-295811 discloses an amorphous porous solid produced by such a sol-gel method and having micropores, mesopores and macropores distributed according to a fractal rule, and it is stated that the amorphous porous solid can be used as an adsorption carrier or as a packing material for chromatography.
However, while the extremely fine and uniform microstructure of the aforementioned conventional zeolite-based crystalline porous solids provides high selectivity and adsorption properties for low molecular weight compounds, the fine microstructure also limits their use as catalysts and adsorbents for compounds with bulky molecular structures or high molecular weight compounds, and their low molecular diffusion rate has also presented a problem from the standpoint of efficiency.
On the other hand, while the aforementioned conventional crystalline mesoporous substances can be utilized as excellent selective catalysts or adsorbents for high molecular weight compounds and their large pore sizes give a high molecular diffusion rate, the relatively large pore sizes and amorphous pore walls also constitute a problem hampering the expression of the specific catalyst and adsorption properties of zeolite and similar materials as molecular sieves.
The aforementioned amorphous porous solids have pores in the full range from micropores to mesopores and even macropores, and can therefore be used as adsorption carriers and catalysts for a wide range of compounds from low molecular weight to high molecular weight, but their drawback is particularly low specific and selective catalytic reactivity and adsorption separation properties for specific compounds.
Thus, none of the conventional porous substances are yet adequate in terms of performance as adsorbents for adsorption separation of specific compounds, and it has been particularly difficult to accomplish adsorption separation of harmful hydrocarbons and global warming-implicated CO2 in exhaust gas in a selective and efficient manner using conventional porous substances as the adsorbents.
It is an object of the present invention, which has been accomplished in light of the aforementioned problems of the prior art, to provide a porous solid for gas adsorption separation, and a gas adsorption separation process employing it, which allow selective and efficient adsorption separation of specific components in gases, such as hydrocarbons and CO2.
As a result of diligent research directed toward achieving this object, the present inventors have discovered that by distributing pore sections (of micropores) in the pore walls themselves separating the pores of crystalline mesoporous substances (mesoporous solids), it is possible to notably improve the adsorption separation performance for specific components in gases, and especially for low molecular weight compounds, thereby accomplishing selective and efficient adsorption separation of the specific components in gases, and the present invention has thus been completed.
The porous solid for gas adsorption separation according to the invention is a porous solid having an X-ray diffraction pattern with at least one peak at a diffraction angle corresponding to a d value of 1 nm or greater; and
having a nitrogen adsorption isotherm measured at liquid nitrogen temperature with at least one section where the change in nitrogen adsorption in terms of the volume of nitrogen under standard conditions is 50 ml/g or greater with a relative vapor pressure change of 0.1 in a relative vapor pressure range of 0.2-0.8;
wherein the porous solid possesses mesopores with a median pore size of 2-50 nm in the pore size distribution curve and pore walls that are porous.
The gas adsorption separation process of the invention is characterized by adsorption separation of components in a gas by contacting the gas with a porous solid having an X-ray diffraction pattern with at least one peak at a diffraction angle corresponding to a d value of 1 nm or greater; and
having a nitrogen adsorption isotherm measured at liquid nitrogen temperature with at least one section where the change in nitrogen adsorption in terms of the volume of nitrogen under standard conditions is 50 ml/g or greater with a relative vapor pressure change of 0.1 in a relative vapor pressure range of 0.2-0.8;
wherein the porous solid possesses mesopores with a median pore size of 2-50 nm in the pore size distribution curve and pore walls that are porous.
The reason for the notably improved adsorption separation performance by the porous solid of the invention has not been established, but the present inventors believe it to be as follows. Specifically, in the porous solid of the invention, the gas molecules diffuse rapidly through the relatively large mesopores into the interior of the pores. Subsequently, the gas molecules are adsorbed and separated by micropores which are formed on the surface of the mesopores (pore walls) and have a pore size corresponding to the molecular size. As this occurs, the gas molecules are selectively adsorbed and separated (filtered) depending on the gas molecule size and the chemical properties of the porous solid surface. The present inventors believe that it is this rapid diffusion and filtering of the gas molecules simultaneously accomplished in the porous solid of the invention that accounts for the selective and efficient adsorption separation of specific components in the gas (such as carbon dioxide or hydrocarbons) from the other components.
The pore walls of the porous solid of the invention are porous with micropores of a mean size of less than 2 nm, and the total volume of these micropores is preferably at least 0.05 ml/g. The total volume of micropores in the porous solid of the invention is preferably at least 10% of the total pore volume. A porous solid with such a large micropore volume tends to exhibit even further improved adsorption separation performance.
In the porous solid of the invention, at least 60% of the total pore volume excluding the micropores is preferably in a range of +40% of the median pore size. A mesopore structure with this degree of homogeneity tends to further improve the selectivity for components in gases.
It is preferred for the median pore size of the mesopores in the porous solid of the invention to be from 3 nm to 30 nm, for the mean size of the micropores to be at least 0.2 nm and less than 2 nm, and for the thickness of the pore walls to be at least 2 nm. A porous solid of this type will tend to exhibit even more satisfactory adsorption separation properties.
The gas adsorption separation process of the invention allows selective and efficient adsorption separation of hydrocarbons, CO2 and the like, as mentioned above, and therefore the target component in the gas is preferably at least one selected from the group consisting of carbon dioxide and hydrocarbons.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.